A long-standing problem in treatment of industrial and municipal waters and waste waters of various types is removal of toxic heavy metals such as copper, zinc, cadmium, lead, nickel, and mercury. Alkaline precipitation is sometimes employed to remove these metals from an aqueous stream, but it is often difficult to reduce the concentration of the metals as below about 10 parts per billion. Part of the removal difficulty may arise from the relatively high solubility of some of the hydroxides. The solubility of several metal hydroxides, as well as the solubility of calcium and iron hydroxide, is shown by their solubility product constants in Table 1. Solubility product constants are the product of the concentration of the ions of a substance in a solution, which is at equilibrium with an excess of that substance, with concentrations expressed as moles per liter.
TABLE 1 ______________________________________ Solubilities of Metal Hydroxides Solubility Product Metal Constant, Solubility from Sulfide Compound Formula K.sub.SP * Hydroxide, ppb ______________________________________ Cadmium Hydroxide Cd(OH)2 5.33 .times. 10.sup.-15 8.4 Lead Hydroxide Pb(OH)2 1.40 .times. 10.sup.-20 0.025 Nickel Hydroxide Ni(OH)2 5.54 .times. 10.sup.-16 1.4 Zinc Hydroxide Zn(OH)2 7.68 .times. 10.sup.-17 0.6 Calcium Hydroxide Ca(OH)2 7.88 .times. 10.sup.-6 113000 Iron Hydroxide Fe(OH)2 4.79 .times. 10.sup.-17 0.4 ______________________________________ *CRC Handbook of Chemistry and Physics, 64th Edition, Page B-219
The solubility from the hydroxide is the amount of that heavy metal that would exist in water in equilibrium with the solid hydroxide compound, provided there were no other sources or sinks for the hydroxyl ion or for the heavy metal ion.
A second method of removing these heavy metals is via ion exchange. This method also has difficulty in removing heavy metal concentrations below about 10 parts per billion (ppb) on a commercial scale. Another difficulty is in the consumption of the removal medium, be it hydroxide ions or available ion exchange sites, by common but innocuous metals such as calcium, iron and the like. While this is easy to monitor and correct for in hydroxide precipitation, it is much more difficult for ion exchange processes.
One technique for thorough removal of heavy metals is precipitation with sulfide. Heavy metal sulfides form very stable and insoluble sulfides, as shown by their solubility product constants in Table 2. The potential for metal removal though the precipitation and collection of sulfides to levels far below the part per billion range is clearly evident.
TABLE 2 ______________________________________ Solubilities of Metal Sulfides Solubility Product Metal Solubility Constant, from Sulfide Sulfide Compound Formula K.sub.SP * ppb ______________________________________ Cadmium Sulfide CdS 1.40 .times. 10.sup.-29 4.2 .times. 10.sup.-7 Copper Sulfide CuS 1.27 .times. 10.sup.-36 .sup. 7.2 .times. 10.sup.-11 Lead Sulfide PbS 9.04 .times. 10.sup.-29 2.0 .times. 10.sup.-6 Mercuric Sulfide HgS 6.44 .times. 10.sup.-53 .sup. 1.6 .times. 10.sup.-18 Nickel Sulfide NiS 1.07 .times. 10.sup.-21 1.9 .times. 10.sup.-3 Zinc Sulfide ZnS 2.93 .times. 10.sup.-25 3.5 .times. 10.sup.-5 ______________________________________ *CRC Handbook of Chemistry and Physics, 73rd Edition, Page 8-43
The principal hindrance to the use of sulfide as a precipitating agent is the fact that the sulfide ion itself is a noxious pollutant in water. It is highly toxic to aquatic life, and also exerts a high oxygen demand as measured by biochemical oxygen demand or chemical oxygen demand. Therefore, the water may well be more toxic after the addition of sulfide, even though many of the heavy metals may have been precipitated out of solution.
The use of ferrous ions to remove a noxious pollutant, cyanide, from cyanide- contaminated water was discussed in U.S. Pat. No. 4,211,646 to Westbrook et al. The Westbrook process involved adding ferrous ions to waters that were contaminated with cyanides, adding a base to increase the pH of the solution to between about 7 to 9, and separating out a sludge comprised of iron cyanide. This patent also discussed the coprecipitation of ferrous ions and nickel, vanadium, and chromium ions in the sludge.